1. Field of the Invention
The present invention relates to electronic devices for use with a universal serial bus (USB). More particularly, the invention relates to circuitry that improves common mode performance of transmitters or receivers, such as in USB-compatible devices, and facilitates connection of devices, such as USB devices, to a host.
2. Description of Related Art
A number of standard interfaces exist for communicating between a host and a device. Referring to FIG. 1, a conventional information handling system 100 is shown. The system 100 makes use of the universal serial bus (xe2x80x9cUSBxe2x80x9d) 125 for connecting a host computer 170 (also known simply as a xe2x80x9chostxe2x80x9d) to a number of devices, known as USB devices, such as a display 135, printer 140, keyboard 145, trackball 150, optical scanner 155, disk drive 160 and other such device 165. Each one of the devices 135, 140, etc. is coupled to the USB 125 via respective ports 130 of the hub 110.
The USB is currently defined by the Universal Serial Bus Specification written and controlled by USB Implementers Forum, Inc., a non-profit corporation founded by the group of companies that developed the USB Specification. In particular, Universal Serial Bus Specification, revision 1.1, dated Sep. 23, 1998 (the xe2x80x9cUSB Specificationxe2x80x9d), Chapter 5 xe2x80x9cUSB Data Flow Model,xe2x80x9d Chapter 7 xe2x80x9cElectrical,xe2x80x9d and Chapter 8 xe2x80x9cProtocol Layerxe2x80x9d are hereby incorporated herein by reference.
According to the USB Specification 1.1, USB devices may include both low speed and full speed devices. Low speed devices transfer data at a transmission rate of 1.5 MHz and full speed devices transfer data at a rate of 12 MHz. Data are transmitted on communication lines. That is, the USB device transmits a differential output signal or receives a differential input signal on these communication lines. In the low speed mode, the differential signal indicates a first logical state, referred to as the xe2x80x9cJxe2x80x9d state, if D+ is at a voltage level below that of Dxe2x88x92, and a second state, the xe2x80x9cKxe2x80x9d state, if D+ is at a voltage level above that of Dxe2x88x92. In the full speed mode, the differential signal indicates a first state, the xe2x80x9cKxe2x80x9d state, if D+ is at a voltage level below that of Dxe2x88x92, and a second state, the xe2x80x9cJxe2x80x9d state, if D+ is at a voltage level above that of Dxe2x88x92. The differential design gives better protection against ground shifts and noise since the received signal level is determined by comparing two voltage levels that are both subject to ground shifts or noise affecting both of the differential signals in a similar manner.
A host 170 detects the presence of a device, such as device 165, on the USB 125 during an attachment phase, while drivers of the port 130 and device are in tri-state. Detection of the attachment is based on a certain connection on the port 130 of a pull-up resistor 210 associated with the device. Likewise, detection of whether the device is operating in low or full speed mode also depends on the pull-up resistor connection.
Referring now to FIG""s 2 and 3, FIG. 2 shows a transmitter 230 of a typical USB device, of the low speed variety, coupled to a receiver 240 on the corresponding port 130. FIG. 3 shows a transmitter 230 of a typical USB device, of the full speed variety, coupled to a receiver 240 of the corresponding port 130. The low speed device (FIG. 2) pull-up resistor 210 is connected between positive voltage contact 213 and the Dxe2x88x92 signal line 212. The full speed device (FIG. 3) has the pull-up resistor 210 connected between the positive voltage contact 213 and the D+ signal line 215. Note that according to the USB Specification, the voltage level of 211 supplying the pull-up resistor is different than that of the specified voltage supplied by the port 130 on the signal line 220 by VBUS. Thus, the resistor, for example, which is conventionally external to integrated circuitry of the USB device in the prior art, is supplied by its own voltage contact 213, and not the VBUS line 220, unless additional circuitry is also included coupled to the line 220 to condition the voltage for supplying the pull-up resistor.
The presence of the pull-up resistor on only the Dxe2x88x92 signal line 212, for low speed peripherals, or on only the D+ signal line 215, for full speed peripherals, introduces an imbalance in the symmetry of the differential signal from the USB transmitter 230, that is, outputs on the signal lines. In other words, due to the resistor, the amplitudes of the signal swings on the signal lines are not the same and the signals do not change at the same rate. This asymmetry is problematic for several reasons, including increases in EMI/RFI radiation, received bit length variation and data stream skew. Aspects of these problems are addressed in U.S. Pat. Nos. 5,905,389 and 5,912,569 (the xe2x80x9cAlleven patentsxe2x80x9d) by introducing a delay circuit in one of the two USB transmitters. While this mitigates the problems, it does not fully eliminate the imbalance in the differential signal arising from the single pull-up resistor.
The presence of the pull-up resistor on one of the communication lines also gives rise to other issues. One issue concerns power consumption by the peripheral. U.S. Pat. No. 6,076,119 (the xe2x80x9cMaemura et al. patentxe2x80x9d) introduces a switch between the pull-up resistor and a terminal voltage, wherein the switch selectively disconnects the pull-up resistor when a device is inoperative. This reduces power consumption, and also simplifies determination by a host computer that a physically connected USB device is inoperative, but it does not address the imbalance in differential signal arising from connection of the single pull-up resistor during operation of the device.
Another issue concerns suitability for use of xe2x80x9csmart cardsxe2x80x9d in connection with a USB. Referring now to FIG. 4, a smart card 400 is shown which has an integrated circuit module (xe2x80x9cICMxe2x80x9d) 420 affixed to a card 410. Although conventional USB peripherals have the USB required pull-up resistor mounted externally, it is problematic to mount a resistor on the surface of a smart card, which is carried in a wallet or purse and repeatedly inserted and removed from a reader. Furthermore, since smart cards compatible with ISO7816 standard are in widespread use in Europe and Asia, legacy issues limit the number of contacts on the smart card which are available for USB applications of smart cards This also gives rise to difficulties in connecting an external resistor to a smart card.
In addition to the above described problems associated with surface mounting and terminal limitations on smart cards, the conventional USB pull-up resistor is also problematic for readers used with smart cards in single-user applications. For relatively centralized applications, such as transactions with payphones, automatic teller machines or point of sale terminals, the number of transactions per smart card reader is high. That is, in these applications each smart card reader is shared by many users, the frequency of transactions per reader is very high, and the cost of the readers is not a major factor. However, smart cards are also useful for widely distributed transactions conducted via the Internet, such as for financial transactions or for logging securely onto a network. For this application, transactions are commonly associated with individual use of computers in homes and offices, and accordingly, smart card readers in this application are used relatively much less frequently, so that the cost per reader is a significant feasibility factor since the solution cost is equal to the smart card reader cost plus the smart card cost.
To overcome the aforementioned problem of USB terminal limitations on ISO7816 compatible smart cards, the use of a conventional USB device""s external pull-up resistor requires voltage conditioning circuitry external to the smart card, as described hereinabove. This pull-up resistor and voltage conditioning circuitry is conventionally located in the smart card reader. As has been stated, this is not an issue for a smart card reader shared by many users, but it is quite problematic for a smart card reader used by a single-user in Internet transactions, because it tends to drive up the cost of the smart card solution.
From the above discussion it should be understood that while advances have been made in USB devices, needs still exist for further improvements which address the problems of EMI/RFI radiation, variation in received bit lengths and skew in the received data stream, all of which arise from imbalance in differential USB signaling due to the USB required pull-up resistor. Furthermore, solutions to these problems and other problems related to the pull-up resistor are particularly difficult for smart cards performing as a USB device, so that the needs are particularly acute in this context.
The foregoing needs are addressed in the present invention. According to a method form of the invention, an apparatus communicates with a host by receiving a voltage at a first voltage level, on a. first one of a number of contacts coupled to an integrated circuit (xe2x80x9cICxe2x80x9d). The contacts and the IC are part of a smart card. The received voltage is conditioned, by voltage conditioning circuitry on the IC., The voltage conditioning circuitry generates an output voltage at a second voltage level for signaling attachment to the host. This is signaled by the voltage conditioning circuitry output pulling up a second contact to the second voltage level, through a resistor of the IC. A signal is also driven on the second contact by a driver on the IC for further communicating to the host. That is, in an embodiment, the driver signal is for communication.
In another aspect of the method, the second contact is pulled up to the second voltage level through a switch on the IC, responsive to the apparatus being powered.
In still another aspect of the method, the second voltage level is decoupled from the second contact by the switch, responsive to a detach indication from control circuitry of the IC.
In yet another aspect, the IC also asserts a second driver signal for differential signal communication to the host, on a third one of the contacts. The voltage conditioning circuitry output voltage is decoupled from the second contact by the switch on the IC, responsive to the apparatus transmitting, i.e., transferring data to the host at the USB full or low speed data rate, to reduce an imbalance for the first and second driver signals. Further, the voltage conditioning circuitry output voltage is re-connected to the second contact by the switch, responsive to termination of the transmitting.
In another embodiment, the voltage conditioning circuitry output voltage is decoupled from the second contact by the switch on the IC, responsive to the apparatus receiving, i.e., transferring data from the host at the USB full or low speed data rate, to reduce an imbalance for signals driven by the host. In this embodiment, the voltage conditioning circuitry output voltage is re-connected to the second contact by the switch, responsive to termination of the receiving.
In another method aspect, receiving the voltage at the first voltage level includes receiving the smart card by a reader having solely passive components, and electrically coupling a connector of the reader to the contacts, for coupling the contacts to the host.
According to an apparatus form of the invention, a device has a driver and outputs for communicating with a host. In a first aspect, the device has voltage conditioning circuitry, and a pull-up resistor and is capable of signaling the host over a bus using the pull-up resistor coupled to a first one of the outputs and a voltage output of the voltage conditioning circuitry.
A switch is included in series with the voltage output, pull-up resistor and the aforementioned first output. The switch is capable of selectively connecting the voltage conditioning circuitry output, through the pull-up resistor, to the first output, responsive to the device being powered by the bus, but not transmitting. This tends to pull up the first output to the voltage level of the voltage conditioning circuitry output, which makes the device capable of being properly detected by the host upon the bus being driven by a host.
In the context of a USB embodiment, the device has two drivers for differential outputs and both the outputs are coupled to an output contact pair. One of this contact pair is for the aforementioned first output. Since the other one of the output contact pair has no corresponding pull-up resistor, it is advantageous to disconnect the pull-up resistor while the device drivers are transmitting, since this results in a more balanced differential output signal, and the benefits of less common-mode noise, reduced EMI/RFI, improved bit lengths and reduced skew in the received bit stream.
The pull-up resistor must at times be connected to the output, however. This is because, as described in the Background hereinabove, for USB applications the host determines if the device is attached to the USB and if the device is low speed or full speed by examining Dxe2x88x92 and D+ signal lines on the USB to which the output terminals of the device may be connected. The invention involves recognition that although the pull-up resistor must be connected for proper detection of the device by the host on the D+ or Dxe2x88x92 lines, the pull-up resistor can advantageously be disconnected when the device is driving those lines.
In an additional aspect, the apparatus of the invention includes an integrated circuit (xe2x80x9cICxe2x80x9d), which is part of a smart card having a number of electrical contacts. (The contacts and the IC are preferably elements of the same integrated circuit module (xe2x80x9cICMxe2x80x9d).) The IC is coupled to the ICM contacts, including a first output of the IC coupled to a first one of the ICM contacts for receiving a voltage supply from a USB port. The IC includes voltage conditioning circuitry coupled to a second one of the ICM contacts through a resistor of the IC.
In still another aspect, the apparatus includes a reader, having a connector for receiving the smart card and coupling connector contacts to the smart card contacts. Electrical components of the reader consist solely of passive components, that is, inactive components having resistance, inductance or capacitance characteristics, but no gain or directional function.
It is an advantage that the pull-up resistor and voltage conditioning circuitry supplying the proper voltage to the pull-up resistor are integrated on the IC, so no contact is required on the smart card to supply the voltage to the resistor. This permits the apparatus to be compatible with the contact configuration of existing smart cards.
It is still another advantage that the resistor being an integrated resistor of the IC eliminates the need for including the pull-up resistor or any voltage conditioning circuitry for the resistor as part of the reader, making the reader more suitable for low cost applications.